1. Field of Art
The invention generally relates to antennas, including for example the combination of RF radiating elements with dielectric construction materials.
2. Description of the Related Art
Traditionally, antennas were positioned, and aimed, to avoid obstructions in order to both minimize losses and preserve the antenna's theoretical, free-space radiation pattern. Increased use of Radio Frequencies (RF) and microwaves in consumer and industrial products have created challenges which require the use of antennas near, and in some cases behind or within, obstructing bodies. Examples include the radiating antennas in AMPS cell phones (800-900 MHz), PCS cell phones (1.8-1.9 GHz), cordless phones, and Wi-Fi devices (2.4-2.5 GHz, 5.7-5.8 GHz). The majority of these devices attempt to concentrate their radiated energy away from the user, with dielectric losses and the antenna's radiation pattern controlled by the manufacturer's selection of housing or radome material and the relative placement of the radiating elements within the housing or radome.
In contrast, antennas which are designed for use within a structure (e.g., for infrastructure deployment) typically must contend with reflections and attenuation due to the intervening walls, ceilings, and other internal structural elements and objects. Attempts to ameliorate the scattering effects of boundaries have generally concentrated on narrowing an antenna's radiation pattern, deploying multiple radiators to illuminate “shadowed” regions, deploying traveling wave or “leaky” antenna structures, utilizing Multiple-Input, Multiple Output (MIMO) antenna signal processing, or utilizing other antenna spatial-temporal techniques to modify the antenna radiation coverage of space. Although each is effective in some set of applications, none act to address the root cause of the problem, specifically, the reflection and attenuation of radiation by the non-transparency of the boundaries themselves.
Furthermore, in cases where the antenna is to be located in a structure (e.g., inside a building) in the most unobtrusive manner possible, it is usually desirable to disguise the antenna, for example to resemble a speaker grille, an HVAC vent, a smoke detector, or a ceiling tile. This visual obscuration is typically accomplished by either placing the antenna behind a structural element (e.g., behind a wall, beneath the floor, or above the ceiling) or by placing camouflaging material around the antenna. One problem with these approaches is that it typically places dielectric material in close proximity to the antenna, which then alters both the antenna's feed-point impedance and its radiation pattern.
FIGS. 1A and 1B illustrate this effect. FIG. 1A shows the free-space radiation pattern 110 of an antenna 120. The antenna 120 is designed specifically to have this radiation pattern 110 and typically will be assumed to have this radiation pattern 110 when installed in the field.
In FIG. 1B, the antenna 120 is installed on one side of a wall 150 (or other obstructing dielectric object). The target volume 160 is on the other side of the wall. For example, the target volume 160 may be the space where people or objects are located and the antenna 120 is installed on the opposite side in order to hide it from view. The wall 150 has two surfaces 155A and 155B. The wall 150 is constructed from a material that acts as a dielectric but has a different dielectric constant than free space. Therefore, the wall 150 distorts the free-space radiation pattern of the antenna 120. FIG. 2 shows both refraction and reflection at each surface 155 as the various rays propagate through the wall 150. This occurs because there is an air-wall interface at each surface 155 and the dielectric constant is different on the two sides of the interface. The superposition of these refractions and reflections produce the actual radiation pattern 127 for the antenna 120 distorted by wall 150.
The resulting antenna detuning, shift of center frequency, and broadening (i.e. loss of directivity) of the radiation pattern 127 is a function of the dielectric constant and thickness of the wall 150, and the relative positions of the intervening material 150, the radiator 120 and the intended target volume 160. While it may be possible to “tune” or otherwise adjust the free-space radiation pattern of the antenna 120 to compensate for this effect in any particular situation, each situation typically will be different. For example, the relative placement, thickness, constituency, dielectric constant or dielectric tensor (if the material is non-homogeneous, chiral, or otherwise polarizing) may differ from one case to the next. Thus, either each antenna will have to be tuned individually to match its situation, which would require a significant amount of work, or certain antennas will not be tuned to match their individual situations. Neither approach is particularly attractive.